In general, a developing device has been known which has a housing, a developing roller, a layer thickness regulation blade and a side seal arranged between the housing and the developing roller at an end portion of the developing roller (see JP-A-2010-54829). By arranging the side seal, a gap is not formed between the housing and the end portion of the developing roller.
Also, the developing device of JP-A-2010-54829 has an upper seal between the housing and the layer thickness regulation blade. The upper seal is arranged so that it is pressed to the layer thickness regulation blade, and is configured so that a gap is not formed between the housing and the layer thickness regulation blade. The upper seal contacts with an end surface of the side seal before the layer thickness regulation blade is mounted. That is, the upper seal is arranged so that a gap is not formed between the upper seal and the side seal.